This invention relates to an apparatus and method for sharpening a stylus and, more particularly, to an apparatus and method for sharpening a diamond cutting stylus which is to be used in a mastering process to produce high density information records, such as video disc records of the type described in U.S. Pat. No. 3,842,194 to J. K. Clemens.
The Clemens' patent discloses a video disc for use with a playback system of the variable capacitance type. In one configuration of the Clemens' system information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. For example, groove widths of approximately 2.6 micrometers and groove depths of about 0.5 micrometers may be used. During playback, capacitive variations between a conductive electrode on a stylus and a conductive property of the disc record are sensed to recover the prerecorded information.
In accordance with the Clemens' format, the video information may be recorded as relatively short (e.g., 0.6-1.6 micrometers) relief variations along the length of the spiral groove. Illustratively, the method of recording may be of a type shown in U.S. Pat. No. 4,044,379 to J. B. Halter. Pursuant to the Halter method, an electromechanically-driven stylus (e.g., of diamond) having a triangular shape, responsive to a combined video and audio signal, records relatively short geometric variations, representative of the time variations of the signal, on a surface of a metal substrate. After the electromechanical recording operation, the recorded surface of the metal substrate has a relief pattern corresponding to that which is desired in the final record. In the replicating process, masters are made from the substrate. Molds are then made from the masters and stampers are made from the molds. The stampers are used in the process of pressing a vinyl record having the desired relief pattern.
In order to record the fine groove and signal structure typically employed in video discs, the recording stylus must cut the groove and signal information without tearing, smearing, chattering, ripping or chipping the surface of the metal substrate during the recording process. The geometric variations along the length of the groove should be a substantially accurate representation of the modulated signals recorded thereon. Bearing in mind the extremely dense nature of the information recorded in the groove structure of the video disc, it will be appreciated that the generation of substrates of acceptable quality is critical in the record mastering process. Any significant incident of machining inperfections in the finished surface results in a substrate which is not acceptable in the manufacturing of records having such a highly dense information content.
It has been discovered that only the hardest materials (e.g., diamond) are acceptable for video recording styli and, further, that the cutting edges of the styli must be extremely sharp to produce substrates to such demanding standards. Sharpening styli used to cut substrates in accordance with these standards presents unique problems to the diamond tool industry.
The stylus structure used to record according to the Halter Method is extremely small. As noted above the signal element and groove structure cut on the master substrate is extremely fine. Further, the video bandwidth (e.g., 0-3 MHz) and modulating frequencies (e.g., up to 6.3 MHz) necessitate the use of an electromechanical cutterhead having a low mass. Generally, a low mass cutterhead has a principle resonant frequency at the upper end of the bandwidth. To provide a stylus suitable for cutting the fine signal structure and having a low mass requires that the stylus structure be miniature in size as compared to styli used in other recording systems, such as audio recording.
Cutting styli for use in electromechanical recording of video information are in short supply and very expensive. The process of manufacturing a new stylus from a rough diamond is tedious. It requires grinding and polishing of several facets. The present invention provides a method and apparatus for use in re-sharpening styli which have become unusable after some period of use in the electromechanical recorder. The size and necessity for such sharp cutting edges affects the methods of handling. During stylus resharpening problems arise with respect to orienting the stylus on the lapping or grinding machine and holding it during the lapping and grinding operation.